The invention relates to machine elements and mechanisms, but more particularly, the invention relates to a flat belt and a belt drive where the belt is of the type with some degree of elasticity in a lengthwise direction of the belt.
U.S. Pat. No. 1,307,731 to Gates discloses a power transmission belt and its method of manufacture where the belt is of the flat, elastomeric type that exhibits some degree of elasticity in a lengthwise direction of the belt. The belt has a tensile member of embedded fibrous material of a square woven fabric cut on a bias and with threads of the fabric forming opposite helical angles in relation to the belt. The tensile member is in the form of a spirally wound volute that forms at least four successive layers. The achievable control over elasticity is substantially limited because: (1) the threads of the bias fabric are interwoven with each other which substantially inhibits their relative movement to each other; (2) the threads of the square woven fabric always form an angle of substantially 90 degrees in relation to each other; and (3) the successive four spiraled plies of square woven fabric are required to "pantograph" and stretch to a greater extent than the previous ply as the belt is bent around pulleys which induces increasing strain in the successive plies.
Flat belts entrained about pulleys whose axis are substantially parallel define a power transmission drive or system and an example of such a drive is referred to in the aforementioned Gates patent. Such a drive is incapable of increasing belt tension to accommodate power transmission loads that require a tension which is greater than the installed belt tension. The belt of the '731 patent has some degree of elasticity in its lengthwise direction and therefore exhibits a spring rate which affects the initial belt installation tension. This is because the pulleys of such a drive are customarily locked and the length of the belt path is constant. Any permanent belt stretch results in a decrease in belt tension. Such drives have limited power transmission capabilities without the aid of another device such as a spring biased belt tensioner with a moveable pulley that engages the belt for the sole purpose of maintaining belt tension.
Flat belts are used in other power transmission drives where the axes of the pulleys are not parallel. A drive where the axes are spaced apart and the plane of the axis are turned in relationship to each other is known as a turned drive. A drive where the axes are turned 90 degrees in relation to each other is known as a "quarter turn drive." Flat belts used on a turned drive must follow a twisted path between the pulleys and the twisting induces various degrees of belt edge strain and, because of this, belts used in these drives usually do not have an embedded tensile member per se. Most of such belts are made of a solid elastomeric material which, in the "vinacular" are referred to as the "rubber band" type. Some flat belts have a covering of bias ply fabric. Both types of such belts have some degree of elasticity and hence, when installed around the pulleys effects the belt installation tension. Some drives of the quarter turn type use a frustoconical pulley for the driver pulley which has the advantage of giving the drive some degree of self tensioning as the belt was able to move up the frustoconical pulley for changes in power transmission load. As a belt moves up to a larger part of the cone of the pulley, the path length of the belt increases which effects an increase in belt tension. The self tensioning features of such a drive are typically limited to the use of a frustoconical pulley and the inherent capability of the belt to effect movement on the frustoconical pulley.